The present invention relates generally to proteinase inhibitors, a precursor thereof and to genetic sequences encoding same.
Nucleotide and amino acid sequences are referred to herein by sequence identity numbers (SEQ ID NOs) which are defined after the bibliography. A general summary of the SEQ ID NOs is provided before the examples.
Throughout this specification and the claims which follow, unless the context requires otherwise, the word xe2x80x9ccomprisexe2x80x9d, or variations such as xe2x80x9ccomprisesxe2x80x9d or xe2x80x9ccomprisingxe2x80x9d, will be understood to imply the inclusion of a stated element or integer or group of elements or integers but not the exclusion of any other element or integer or group of elements or integers.
Several members of the families Solanaceae and Fabaceae accumulate serine proteinase inhibitors in their storage organs and in leaves in response to wounding (Brown and Ryan, 1994; Richardson, 1977). The inhibitory activities of these proteins are directed against a wide range of proteinases of microbial and animal origin, but rarely against plant proteinases (Richardson, 1977). It is believed that these inhibitors are involved in protection of the plants against pathogens and predators. In potato tubers and legume seeds, the inhibitors can comprise 10% or more of the stored proteins (Richardson, 1977), while in leaves of tomato and potato (Green and Ryan, 1972), and alfalfa (Brown and Ryan, 1984), proteinase inhibitors can accumulate to levels of 2% of the soluble protein within 48 hours of insect attack, or other types of wounding (Brown and Ryan, 1984; Graham et al., 1986). High levels of these inhibitors (up to 50% of total soluble protein) are also present in unripe fruits of the wild tomato, Lycopersicon peruvianum (Pearce et al., 1988).
There are two families of serine proteinase inhibitors in tomato and potato (Ryan, 1984). Type I inhibitors are small proteins (monomer Mr 8100) which inhibit chymotrypsin at a single reactive site (Melville and Ryan, 1970; Plunkett et al., 1982). Inhibitors of the type II family generally contain two reactive sites, one of which inhibits chymotrypsin and the other trypsin (Bryant et al., 1976; Plunkett et al., 1982). The type II inhibitors have a monomer Mr of 12,300 (Plunkett et al., 1982). Proteinase inhibitor I accumulates in etiolated tobacco (Nicotiana tabacum) leaves (Kuo et al., 1984), and elicitors from Phytophthora parasilica var. nicotianae were found to induce proteinase inhibitor I accumulation in tobacco cell suspension cultures (Rickauer et al., 1989).
There is a need to identify other proteinase inhibitors and to investigate their potential use in the development of transgenic plants with enhanced protection against pathogens and predators. In accordance with the present invention, genetic sequences encoding a proteinase inhibitor precursor have been cloned. The precursor has multi-proteinase inhibitor domains and will be useful in developing a range of transgenic plants with enhanced proteinase inhibitor expression. Such plants will have enhanced protective properties against pathogens and predators. The genetic constructs of the present invention will also be useful in developing vaccines for ingestion by insects which are themselves predators or which act as hosts for plant pathogens. The recombinant precursor or monomeric inhibitors will also be useful in topical sprays and in assisting animals in feed digestion.
Accordingly, one aspect of the present invention relates to a nucleic acid molecule comprising a sequence of nucleotides which encodes or is complementary to a sequence which encodes a type II serine proteinase inhibitor (PI) precursor from a plant wherein said precursor comprises at least three PI monomers and wherein at least one of said monomers has a chymotrypsin specific site and at least one other of said monomers has a trypsin specific site.
The xe2x80x9cnucleic acid moleculexe2x80x9d of the present invention may be RNA or DNA (eg cDNA), single or double stranded and linear or covalently closed. The nucleic acid molecule may also be genomic DNA corresponding to the entire gene or a substantial portion thereof or to fragments or derivatives thereof. The nucleotide sequence may correspond to the naturally occurring nucleotide sequence of the genomic or cDNA clone or may contain single or multiple nucleotide substitutions, deletions and/or additions thereto. All such variants in the nucleic acid molecule either retain the ability to encode at least one monomer or active part thereof or are useful as hybridisation probes or polymerase chain reaction (PCR) primers for the same or similar genetic sequences in other sources.
Preferably, the PI precursor comprises at least four, more preferably at least five and even more preferably at least six PI monomers. Still more preferably, the PI precursor further comprises a signal sequence. The PI precursor of the present invention preferably comprises amino acid sequences which are process sites for cleavage into individual monomers.
The term xe2x80x9cprecursorxe2x80x9d as used herein is not intended to place any limitation on the utility of the precursor molecule itself or a requirement that the molecule first be processed into monomers before PI activity is expressed. The precursor molecule has PI activity and the present invention is directed to the precursor and to the individual monomers of the precursor.
Furthermore, the present invention extends to a nucleic acid molecule comprising a sequence of nucleotides which encodes or is complementary to a sequence which encodes a hybrid type II serine PI precursor wherein said precursor comprises at least two monomers from different PIs. The at least two monomers may be modified such as being unable to be processed into individual monomers or may retain the ability to be so processed. Preferably, at least one of said monomers has a chymotrypsin specific site and the other of said monomers has a trypsin specific site. Preferably there are at least three monomers, more preferably at least four monomers, still more preferably at least five monomers and yet still more preferably at least six monomers wherein at least two are from different PIs. In a most preferred embodiment, at least one of said monomers is a thionin. Such hybrid PI precursors and/or monomers thereof are particularly useful in generating molecules which are xe2x80x9cmulti-valentxe2x80x9d in that they are active against a range of pathogens and predators such as both fungi and insects. Accordingly, reference herein to xe2x80x9cPI precursorxe2x80x9d includes reference to hybrid molecules.
The present invention is exemplified by the isolation of the subject nucleic acid molecule from Nicotiana alata which has the following nucleotide sequence (SEQ ID NO. 1) and a corresponding amino acid sequence (SEQ ID NO. 3):
This is done, however, with the understanding that the present invention extends to an equivalent or substantially similar nucleic acid molecule from any other plant. By xe2x80x9cequivalentxe2x80x9d and xe2x80x9csubstantially similarxe2x80x9d is meant at the level of nucleotide sequence, amino acid sequence, antibody reactivity, monomer composition and/or processing of the precursor to produce monomers. For example, a nucleotide sequence having a percentage sequence similarity of at least 55%, such as about 60-65%, 70-75%, 80-85% and over 90% when compared to the sequence of SEQ ID NO. 1 would be considered xe2x80x9csubstantially similarxe2x80x9d to the subject nucleic acid molecule provided that such a substantially similar sequence encodes a PI precursor having at least three monomers and preferably four, five or six monomers as hereinbefore described.
In a particularly preferred embodiment, the nucleic acid molecule further encodes a signal sequence 5xe2x80x2 to the open reading frame and/or a nucleotide sequence 3xe2x80x2 of the coding region providing a full nucleotide sequence as follows (SEQ ID NO. 2):
Accordingly, a preferred embodiment of the present invention provides a nucleic acid molecule comprising a sequence of nucleotides as set forth in SEQ ID NO. 1 or 2 which encodes or is complementary to a sequence which encodes a type II serine PI precursor from Nicotiana alata or having at least 55% similarity to said precursor or at least one domain therein wherein said precursor comprises a signal peptide and at least five monomers and wherein one of said monomers has a chymotrypsin specific site and four of said monomers have trypsin specific sites.
In still a more preferred embodiment, the nucleic acid molecule is a cDNA molecule and comprises a nucleotide sequence generally as set forth in SEQ ID NO. 1 or 2 or being substantially similar thereto as hereinbefore defined to the whole of said sequence or to a domain thereof.
Another aspect of the present invention is directed to a nucleic acid molecule comprising a sequence of nucleotides which encodes or is complementary to a sequence which encodes a single type II serine PI having either a chymotrypsin specific site or a trypsin specific site and wherein said PI is a monomer of a precursor PI having at least three monomers of which at least one of said monomers has a chymotrypsin site and the other of said monomers has a trypsin site. Preferably, however, the precursor has four, five or six monomers and is as hereinbefore defined.
In its most preferred embodiment, the plant is N. alata (Link et Otto) having self-incompatibility genotype S1S3, S3S3 or S6S6, and the nucleic acid molecule is isolatable from or complementary to genetic sequences isolatable from stigmas and styles of mature plants. The corresponding mRNA is approximately 1.4 kb and the cDNA has six conserved domains wherein the first two domains are 100% identical and contain chymotrypsin-specific sites (Leu-Asn). The third, fourth and fifth domains share 95-98% identity and have sites specific for trypsin (Arg-Asn). A sixth domain which also has a trypsin specific site has less identity to the third, fourth and fifth domains (79-90%) due mainly to a divergent 3xe2x80x2 sequence (see Table 1). The preferred PI inhibitor of the present invention has a molecular weight of approximately 42-45 kDa with an approximately 29 amino acid signal sequence.
The N-terminal sequence of the monomeric PI is represented in each of the six repeated domains in the predicted sequence of the PI precursor protein. Thus, it is likely that the PI precursor protein is cleaved at six sites to produce seven peptides. Six of the seven peptides, peptides 2, 3, 4, 5, 6 and 7 (FIG. 1, residues 25-82 [SEQ ID NO. 5], 83-140 [SEQ ID NO. 6], 141-198 [SEQ ID NO. 7], 199-256 [SEQ ID NO. 8], 257-314 [SEQ ID NO. 9] and 315-368 [SEQ ID NO. 9], respectively), would be in the same molecular weight range as the monomeric PI (about 6 kDa) and would have the same N-terminal sequence. Peptide 7 does not contain a consensus site for trypsin or chymotrypsin. Peptide 1 (residues 1-24 [SEQ ID NO. 4], FIG. 1) is smaller than 6 kD, has a different N-terminus and was not detected in a purified monomeric PI preparation. It could be envisaged that peptide 1 and peptide 7 would form a functional proteinase inhibitor with the inhibitory site on peptide 1 held in the correct conformation by disulphide bonds formed between the two peptides.
Although not intending to limit the present invention to any one hypothesis, the PI precursor may be processed by a protease responsible, for example, for cleavage of an Asn-Asp linkage, to produce the bioactive monomers. More particularly, the protease sensitive sequence is R1-X1-X2-Asn-Asp-R2 where R1, R2, X1 and X2 are defined below. The discovery of such a sequence will enable the engineering of peptides and polypeptides capable of being processed in a plant by cleavage of the protease sensitive sequence. According to this aspect of the present invention there is provided a protease sensitive peptide comprising the amino acid sequence:
-X1-X2-Asn-Asp-xe2x80x83xe2x80x83(SEQ ID NO:15)
wherein X1 and X2 are any amino acid but are preferably both Lys residues. The protease sensitive peptide may also be represented as:
R1-X1-X2-Asn-Asp-R2
wherein X1 and X2 are preferably the same and are preferably both Lys residues and wherein R1 and R2 are the same or different, any D or L amino acid, a peptide, a polypeptide, a protein, or a non-amino acid moiety or molecule such as, but not limited to, an alkyl (eg methy, ethyl), substituted alkyl, alkenyl, substituted alkenyl, acyl, dienyl, arylalkyl, arylalkenyl, aryl, substituted aryl, heterocyclic, substituted heterocyclic, cycloalkyl, substituted cycloalkyl, halo (e.g. Cl, Br, I, F), haloalkyl, nitro, hydroxy, thiol, sulfonyl, carboxy, alkoxy, aryloxy and alkylaryloxy group and the like as would be apparent to one skilled in the art. By alkyl, substitued alkyl, alkenyl and substituted alkenyl and the like is meant to encompass straight and branched molecules, lower (C1-C6) and higher (more than C6) derivatives. The term xe2x80x9csubstitutedxe2x80x9d includes all the substituents set forth above.
In its most preferred embodiment, the protease sensitive peptide is:
R1-X1-X2-Asn-Asp-R2
wherein R1 and R2 are the same or different and are peptides or polypeptides and wherein X1 and X2 are both Lys residues.
Such a protease sensitive peptide can be placed between the same or different monomers so that upon expression in a suitable host or in vitro, the larger molecule can be processed to the peptides located between the protease sensitive peptides.
The present invention also extends to a nucleic acid molecule comprising a sequence of nucleotides which encodes or is complementary to a sequence which encodes a protease sensitive peptide comprising the sequence:
-X1-X2-Asn-Asp-xe2x80x83xe2x80x83(SEQ ID NO:15)
wherein X1 and X2 are preferably the same and are most preferably both Lys residues. Such a nucleic acid molecule may be part of a larger nucleotide sequence encoding, for example, a precursor polypeptide capable of being processed via the protease sensitive sequence into individual peptides or monomers.
The protease sensitive peptide of the present invention is particularly useful in generating poly and/or multi-valent xe2x80x9cprecursorsxe2x80x9d wherein each monomer is the same or different and directed to the same or different activities such as anti-viral, anti-bacterial, anti-fungal, anti-pathogen and/or anti-predator activity.
Although not wishing to limit this aspect of the invention to any one hypothesis or proposed mechanism of action, it is believed that the protease acts adjacent the Asn residue as more particularly between the Asn-Asp residues.
The present invention extends to an isolated type II serine PI precursor from a plant wherein said precursor comprises at least three PI monomers and wherein at least one of said monomers has a chymotrypsin specific site and at least one other of said monomers has a trypsin specific site. Preferably, the PI precursor has four, five or six monomers and is encoded by the nucleic acid molecule as hereinbefore described. The present invention also extends to the individual monomers comprising the precursor. The present invention also extends to a hybrid recombinant PI precursor molecule comprising at least two monomers from different PIs as hereinbefore described.
The isolated PI or PI precursor may be in recombinant form and/or biologically pure. By xe2x80x9cbiologically purexe2x80x9d is meant a preparation of PI, PI precursor and/or any mixtures thereof having undergone at least one purification step including ammonium sulphate precipitation, Sephadex chromatography and/or affinity chromatography. Preferably, the preparation comprises at least 20% of the PI, PI precursor or mixture thereof as determined by weight, activity antibody, reactivity and/or amino acid content. Even more preferably, the preparation comprises 30-40%, 50-60% or at least 80-90% of PI, PI precursor or mixture thereof.
The PI or its precursor may be naturally occurring or be a variant as encoded by the nucleic acid variants referred to above. It may also contain single or multiple substitutions, deletions and/or additions to its amino acid sequence or to non-proteinaceous components such as carbohydrate and/or lipid moieties.
The recombinant and isolated PI, PI precursor and mixtures thereof are useful as laboratory reagents, in the generation of antibodies, in topically applied insecticides as well as orally ingested insecticides.
The recombinant PI or PI precursor may be provided as an insecticide alone or in combination with one or more carriers or other insecticides such as the BT crystal protein.
The PI of the present invention is considered to have a defensive role in organs of the plant, for example, the stigma, against the growth or infection by pests and pathogens such as fungi, bacteria and insects. There is a need, therefore, to develop genetic constructs which can be used to generate transgenic plants capable of expressing the PI precursor where this can be processed into monomers of a monomeric PI itself.
Accordingly, another aspect of the present invention contemplates a genetic construct comprising a nucleic acid molecule comprising a sequence of nucleotides which encodes or is complementary to a sequence which encodes a type II serine PI precursor or monomer thereof from a plant wherein said precursor comprises at least three PI monomers and wherein at least one of said monomers has a chymotrypsin specific site and at least one of said other monomers has a trypsin specific site and said genetic sequence further comprises expression means to permit expression of said nucleic acid molecule, replication means to permit replication in a plant cell or, alternatively, integration means, to permit stable integration of said nucleic acid molecule into a plant cell genome. Preferably, the expression is regulated such as developmentally or in response to infection such as being regulated by an existing PI regulatory sequence. Preferably, the expression of the nucleic acid molecule is enhanced to thereby provide greater endogenous levels of PI relative to the levels in the naturally occurring plant. Alternatively, the PI precursor cDNA of the present invention is usable to obtain a promoter sequence which can then be used in the genetic construct or to cause its manipulation to thereby permit over-expression of the equivalent endogenous promoter. In another embodiment the PI precursor is a hybrid molecule as hereinbefore described.
Yet another aspect of the present invention is directed to a transgenic plant carrying the genetic sequence and/or nucleic acid molecule as hereinbefore described and capable of producing elevated, enhanced or more rapidly produced levels of PI and/or PI precursor or hybrid PI precursor when required. Preferably, the plant is a crop plant or a tobacco plant but other plants are usable where the PI or PI precursor nucleic acid molecule is expressable in said plant. Where the transgenic plant produces PI precursor, the plant may or may not further process the precursor into monomers. Alternatively, the genetic sequence may be part of a viral or bacterial vector for transmission to an insect to thereby control pathogens in insects which would consequently interrupt the transmission of the pathogens to plants.
In still yet another aspect of the present invention, there is provided antibodies to the PI precursor or one or more of its monomers. Antibodies may be monoclonal or polyclonal and are useful in screening for PI or PI precursor clones in an expression library or for purifying PI or PI precursor in a fermentation fluid, supernatant fluid or plant extract.
The genetic constructs of the present invention can also be used to populate the gut of insects to act against the insect itself or any plant pathogens therein or to incorporate into the gut of animals to facilitate the digestion of plant material.
The present invention is further described by reference to the following non-limiting Figures and Examples.